1. Technical Field
The present invention relates to an inflatable vehicle occupant restraint system having ignitable gas generating material for inflating an airbag. In particular, the present invention relates to an igniter for igniting gas generating material.
2. Description of the Prior Art
Igniters for igniting gas generating material in inflatable vehicle occupant restraint systems are known. One such igniter is disclosed in U.S. Pat. No. 4,561,675. In U.S. Pat. No. 4,561,675, the igniter and gas generating material are supported in an aluminum housing having a mechanical strength which degrades when heated to an elevated temperature. If the gas generating material is ignited when the aluminum housing is at a temperature of about 650.degree. F., the housing may burst and pieces of the housing may fly in all directions. To avoid this problem, U.S. Pat. No. 4,561,675 uses an auto-ignition material located in a container adjacent the igniter. The auto-ignition material is a smokeless powder and ignites at approximately 350.degree. F. The auto-ignition material ignites the booster material which, in turn, ignites the gas generating material or the auto-ignition material ignites the gas generating material directly. At 350.degree. F. the aluminum housing has sufficient strength to retain the pressure resulting from the generation of gas by the gas generating material so that the housing does not rupture or fragment. The structure of U.S. Pat. No. 4,561,675 has a disadvantage, however, in that the auto-ignition material is in a separate container which adds cost and extra steps to the assembly of the inflator.
Another known igniter for igniting gas generating material in a vehicle occupant restraint system includes a bridgewire connected to electrically conductive leads. The leads are operatively connected with a power source. The bridgewire is coated with lead styphnate (lead trinitroresorcinate). Lead styphnate is an auto-ignition material. However, there are disadvantages in using lead styphnate. Among these disadvantages are that the auto-ignition temperature of 464.degree. F. of lead styphnate is higher than the auto-ignition temperature of 350.degree. F. of the smokeless powder used in U.S. Pat. No. 4,561,675 and lead styphnate is very difficult to handle safely when applied to the bridgewire.